字符串查找，不使用indexOf，substr,正则，substring,contain,slice等现成的方法

var str = '12', strAll = '4512563';

function isContain(a, b) {
     var aL = a.length,bL = b.length,index = -1;
     for (var i = 0; i < bL - aL; i++) {
         if (b.slice(i, i + aL) == str) {
             index = i
          }
     }
     return index;
}
isContain(str, strAll)